The present invention relates generally to vehicle braking and controllability control systems, commonly referred to as stability control systems, and specifically to a braking and controllability control method and system for a vehicle with regenerative braking that optimizes energy recovery while reducing vehicle oversteer and understeer.
The need to reduce fossil fuel consumption and emissions in automobiles and other vehicles predominately powered by internal combustion engines (ICEs) is well known. Vehicles powered by electric motors attempt to address these needs. Another alternative solution is to combine a smaller ICE with electric motors into one vehicle. Such vehicles combine the advantages of an ICE vehicle and an electric vehicle and are typically called hybrid electric vehicles (HEVs). See generally, U.S. Pat. No. 5,343,970 to Severinsky.
The HEV is described in a variety of configurations. Many HEV patents disclose systems where an operator is required to select between electric and internal combustion operation. In other configurations, the electric motor drives one set of wheels and the ICE drives a different set.
Other, more useful, configurations have developed. For example, a series hybrid electric vehicle (SHEV) configuration is a vehicle with an engine (most typically an ICE) connected to an electric motor called a generator. The generator, in turn, provides electricity to a battery and another motor, called a traction motor. In the SHEV, the traction motor is the sole source of wheel torque. There is no mechanical connection between the engine and the drive wheels. A parallel hybrid electrical vehicle (PHEV) configuration has an engine (most typically an ICE) and an electric motor that work together in varying degrees to provide the necessary wheel torque to drive the vehicle. Additionally, in the PHEV configuration, the motor can be used as a generator to charge the battery from the power produced by the ICE.
A parallel/series hybrid electric vehicle (PSHEV) has characteristics of both PHEV and SHEV configurations and is sometimes referred to as a xe2x80x9csplitxe2x80x9d parallel/series configuration. In one of several types of PSHEV configurations, the ICE is mechanically coupled to two electric motors in a planetary gear-set transaxle. A first electric motor, the generator, is connected to a sun gear. The ICE is connected to a carrier gear. A second electric motor, a traction motor, is connected to a ring (output) gear via additional gearing in a transaxle. Engine torque can power the generator to charge the battery. The generator can also contribute to the necessary wheel (output shaft) torque if the system has a one-way clutch. The traction motor is used to contribute wheel torque and to recover braking energy to charge the battery. In this configuration, the generator can selectively provide a reaction torque that may be used to control engine speed. In fact, the engine, generator motor and traction motor can provide a continuous variable transmission (CVT) effect. Further, the HEV presents an opportunity to better control engine idle speed over conventional vehicles by using the generator to control engine speed.
The desirability of combining an ICE with electric motors is clear. There is great potential for reducing vehicle fuel consumption and emissions with no appreciable loss of vehicle performance or driveability. The HEV allows the use of smaller engines, regenerative braking, electric boost, and even operating the vehicle with the engine shutdown. Nevertheless, new ways must be developed to optimize the HEV""s potential benefits.
One such area of HEV development is optimizing the braking and stability system of the HEV or any other type of vehicle using regenerative braking technology. Regenerative braking (regen) captures the kinetic energy of the vehicle as it decelerates. In conventional vehicles, kinetic energy usually dissipates as heat in a vehicle""s brakes or engine during deceleration. Regen converts the captured kinetic energy through a generator into electrical energy in the form of a stored charge in a vehicle""s battery. This stored energy is later used to power the electric motor. Consequently, regen also reduces fuel usage and emission production. In certain vehicle configurations, the engine can be disconnected from the rest of the powertrain thereby allowing more of the kinetic energy to be converted into stored electrical energy.
On most vehicles with regenerative braking, the regenerative braking torque is applied to, or predominantly to, the wheels of only one axle. When regenerative braking is applied to the wheels of only one axle, non-regenerative braking methods may be used at the wheels of the other axles. The desire to recover energy through regenerative braking can result in different braking torques being applied to the wheels of the different axles. The difference between the braking torques can cause unbalanced braking that may degrade vehicle controllability. Degraded controllability can be in the form of either reduced stability or reduced steerability. For example, when excessive regenerative braking torque is applied at the front axle, as in front wheel drive vehicles, the ability of the front wheels to steer the vehicle may be reduced. The reduced steerability is a condition known as understeer. When excessive regenerative braking torque is applied at the rear axle, as in rear wheel drive vehicles, the lateral friction of the rear tires may be reduced. The reduced stability is a condition known as oversteer. Both of these effects, understeer due to excessive levels of regenerative braking at the front axle and oversteer due to excessive levels of regenerative braking at the rear axle, can become greater on low friction surfaces such as ice and snow. The requirement for controllability on low friction surfaces typically forces regenerative braking levels to be reduced, resulting in a corresponding loss of energy recovery.
There are HEV patents directed to control of regenerative braking functions in various driving conditions. Koga et al. (U.S. Pat. No. 6,033,041) describes a regenerative braking control system for an electric vehicle where the regenerative braking varies as a function of vehicle inclination. Okamatsu (U. S. Pat. No. 4,335,337) describes a control system for an electric powered vehicle. This invention attempts to improve tire grip performance by adjusting the frequency of the rotations of the induction motors based on the slip frequency of the vehicle without regard to regenerative braking.
Ohtsu et al., (U.S. Pat. No. 5,476,310) also attempts to improve braking performance through the cooperation of mechanical anti-lock brakes and regenerative braking. This invention regulates excessive braking force and slip with a controller using a predetermined slip ratio. Other inventions also attempt to regulate excessive slip. See Asa et al. (U.S. Pat. No. 5,654,887) and Kidston et al. (U.S. Pat. No. 5,615,933). Unfortunately, while these inventions do reduce excessive slip, they do not provide an adequate level of stability because they focus mainly on the maximization of straight line stopping.
Asanuma et al. (U.S. Pat. No. 5,318,355), describes a switchover mode from a regenerative or friction braking mode of operation. Unfortunately, this invention is susceptible to false activation of the mode switchover.
Thus the ability to distribute brake torque between regenerative and non-regenerative braking while optimizing energy recovery and vehicle controllability constitutes and unmet need in the art.
Accordingly, the present invention provides a method and system to control braking and improve controllability of a vehicle equipped with regenerative braking. The invention can provide regenerative braking while reducing understeer and oversteer while not significantly reducing energy recovery, even on low friction surfaces. The invention continuously detects oversteer and understeer and correspondingly balances regenerative braking torque with conventional non-regenerative braking torque if controllability decreases.
In accordance with an important aspect of the present invention regenerative braking torque is applied to the wheels of at least one axle, mechanical friction or other non-regenerative brakes known in the art being connected to the wheels of another axle. In accordance with the invention the regenerative braking and non-regenerative braking forces are applied to the wheel of different axles. The vehicle has a controller having the microprocessor hardware and software to receive and evaluate sensor input of brake position and wheel speed of each wheel and to activate a generator motor which varies non-regenerative and regenerative braking in optimum proportions between the front axle and rear axle for maximum energy recovery and vehicle controllability. Vehicle controllability is determined based on at least one actual vehicle controllability value and at least one predetermined target value. Regenerative braking is adjusted so as to maintain the actual vehicle controllability value within the predetermined target value. The controller can be a simple proportional-integral-derivative feedback type of controller.
The present invention can reduce oversteer in vehicle configurations where the generator motor provides variable amounts of regenerative braking torque to the wheels of the rear axle and where the non-regenerative brakes are connected to the wheels of the front axle. The sensor input preferably includes data related to steering angle, yaw rate and lateral acceleration. In addition, vehicle controllability determinations, such as oversteer, can be based on data related to longitudinal wheel slip ratio, tire slip angle, and yaw rate. Steering angle can be determined from steering wheel position, steerable wheel position or a time-filtered determination of steering angle. The present invention can also be configured to reduce understeer in front wheel drive vehicles.